1. Field of the Invention
The present invention relates to a semiconductor device having a three-dimensional structure, and more particularly to a three-dimensional semiconductor device which is well-suited for attaining a high density of integration.
2. Description of the Prior Art
As is well known, most conventional semiconductor devices have been such that elements such as transistors are formed in the surface region of a semiconductor substrate. Even an LSI having a high density of integration has been similarly formed in the surface region of a semiconductor substrate.
Since, however, the number of elements such as transistors to be formed in the surface region of a single semiconductor substrate has strikingly increased in recent years, it has gradually become difficult to rapidly increase the number of the elements still more in the future so as to enhance the density of integration of a semiconductor integrated circuit over great strides.
In order to solve this problem, there has been proposed the so-called three-demensional semiconductor device wherein insulator films and single-crystal semiconductor films are alternately stacked on a semiconductor substrate and wherein a large number of elements are formed in each of the semiconductor films.
For example, there has been proposed a three-dimensional semiconductor device of a structure wherein p-channel MOS transistors are formed on a substrate side, stacked Si and insulator films are formed thereon by the use of the well-known SOI (Silicon On Insulator) technique, and n-channel MOS transistors are formed by utilizing the interfaces of the Si and SiO.sub.2 films. That is, this semiconductor device employs a single gate as the common gates of the upper and lower MOS transistors, thereby making it possible to simultaneously operate the p-channel and n-channel MOS transistors by the single gate used in common. (Gibbons et al., IEEE, EDL-1, 117, 1980)
Since, however, the semiconductor devices of three-dimensional structures are not long-established, a novel structure must be found out in order to fabricate a semiconductor device which has a still higher density of integration and which affords new functions.